And it's in... Oregon? Sort of. In an interview with Smithsonian Magazine, "The Simpsons" creator Matt Groening offers the following tidbit:
OK, why do the Simpsons live in a town called Springfield? Isn’t that a little generic?
Springfield was named after Springfield, Oregon. The only reason is that when I was a kid, the TV show “Father Knows Best” took place in the town of Springfield, and I was thrilled because I imagined that it was the town next to Portland, my hometown. When I grew up, I realized it was just a fictitious name. I also figured out that Springfield was one of the most common names for a city in the U.S. In anticipation of the success of the show, I thought, “This will be cool; everyone will think it’s their Springfield.” And they do.
Springfield, Oregon, population 59,403, is located next to Eugene to the west, and while that's a solid origin story for one of television's most famously generic town settings, we all know Springfield really lives in our hearts and minds and some geographically impossible location, contradictory hints about which have been dropped throughout the show's 23-season run.
In the 2007 movie, for instance, Ned Flanders tells Bart that the state in which Springfield's located is bordered on by Ohio, Nevada, Maine, and Kentucky. Oregon should be so lucky.